An example of an environment in which the present invention is intended to operate is an arrangement for continuously casting molten metal directly into strip, e.g., steel strip. Such an apparatus typically comprises a pair of horizontally spaced rolls mounted for rotation in opposite rotational senses about respective horizontal axes. The two rolls define a horizontally disposed, vertically extending gap therebetween for receiving the molten metal. The gap defined by the rolls tapers in a downward direction. The rolls are cooled, and in turn cool the molten metal as the molten metal descends through the gap.
The gap has horizontally spaced, open opposite sides adjacent the ends of the two rolls. The molten metal is unconfined by the rolls at the open ends of the gap. To prevent molten metal from escaping outwardly through the open ends of the gap, mechanical dams or seals have been employed.
Mechanical dams have drawbacks because the dam is in physical contact with both the rotating rolls and the molten metal. As a result, the dam is subject to wear, leaking and breakage and can cause freezing and large thermal gradients in the molten metal. Moreover, contact between the mechanical dam and the solidifying metal can cause irregularities along the edges of metal strip cast in this manner, thereby offsetting the advantages of continuous casting over the conventional method of rolling metal strip from a thicker, solid entity.
The advantages obtained from the continuous casting of metal strip, and the disadvantages arising from the use of mechanical dams or seals are described in more detail in Praeg U.S. Pat. No. 4,936,374 and in Lari, et al. U.S. Pat. No. 4,974,661, and the disclosures of each of these patents are incorporated herein by reference.
To overcome the disadvantages inherent in the employment of mechanical dams or seals, efforts have been made to contain the molten metal at the open end of the gap between the rolls by employing an electromagnet having a core encircled by a conductive coil through which an alternating electric current flows and having a pair of magnet poles located adjacent the open end of the gap. The magnet is energized by the flow of alternating current through the coil, and the magnet generates an alternating or time-varying magnetic field, extending across the open end of the gap, between the poles of the magnet. The magnetic field can be either horizontally disposed or vertically disposed, depending upon the disposition of the poles of the magnet. Examples of magnets which produce a horizontal field are described in the aforementioned Praeg U.S. Pat. No. 4,936,374; and examples of magnets which produce a vertical magnetic field are described in the aforementioned Lari, et al. U.S. Pat. No. 4,974,661.
The alternating magnetic field induces eddy currents in the molten metal adjacent the open end of the gap, creating a repulsive force which urges the molten metal away from the magnetic field generated by the magnet and thus away from the open end of the gap.
The static pressure force urging the molten metal outwardly through the open end of the gap between the rolls increases with increased depth of the molten metal, and the magnetic pressure exerted by the alternating magnetic field must be sufficient to counter the maximum outward pressure exerted on the molten metal. A more detailed discussion of the considerations described in the preceding sentence and of the various parameters involved in those considerations are contained in the aforementioned Praeg and Lari, et al. U.S. Patents.
With horizontally disposed electromagnetic fields, the prior art achieves magnetic confinement of the sidewall of molten metal at the open end of the gap by providing a low reluctance flux path near the end of each roll (the rim portion of the roll). The apparatus of the prior art comprises an electromagnet for generating an alternating magnetic field that is applied, via the low reluctance rim portions of the rolls, to the sidewall of the molten metal contained by the rolls. For efficient application of the magnetic field, each magnet pole must extend axially, relative to the rolls, very close to the end of a respective roll to be next to the low reluctance rim portion of the roll and separated from this rim portion by only a small radial air gap. For efficient operation, the low reluctance flux path in the rim portion of a roll usually is formed from highly permeable magnetic material.
The prior art electromagnetic confinement methods and apparatuses have several drawbacks:
(1) The peak flux density obtainable is limited by saturation of the highly permeable magnetic material in the rim portions of the rolls, or, in applications where the rim portions do not contain permeable magnetic material, by saturation of the poles of the electromagnet. The state of the art, utilizing thin laminations of grain-oriented silicon steel, limits the horizontal field to approximately 18 kG (Kilogauss). This in turn limits the height of the molten metal pool that can be contained electromagnetically. In addition, at these high flux densities, the heat losses in both the roll laminations, and in the laminations of the magnetic poles near the nip, become excessive; for 0.002 inch (0.051 mm) laminations operating at 18 kG and 3 kHz (KiloHertz), losses are about 300 Watts per pound (660.8 W/kg); PA0 (2) The low reluctance rim portions of the rolls are difficult to cool, resulting in a more complicated and expensive roll design; PA0 (3) The pool of molten metal causes thermal expansion of the rolls which in turn causes stress and strain and/or spatial changes in the low reluctance flux path of the roll rims, altering their reluctance and with it, the performance of the electromagnetic containment; and PA0 (4) In case of a disturbance in the molten metal feed system, or a power failure to the electromagnet, the molten metal (at .apprxeq.1540.degree. C., for steel) will contact the low reluctance rim portion, necessitating a rim design resistant to the high temperature of the molten metal. A high temperature design for the roll rims impairs its low magnetic reluctance, and, most likely increases its cost of manufacture.
Another expedient for horizontal containment of molten metal at the open end of a gap between a pair of members, e.g., rolls, is to locate, adjacent the open end of the gap, a coil through which an alternating current flows. This causes the coil to generate a magnetic field which induces eddy currents in the molten metal adjacent the open end of the gap resulting in a repulsive force similar to that described above in connection with the magnetic field generated by an electromagnet. Embodiments of this type of expedient are described in Olsson U.S. Pat. No. 4,020,890, and the disclosure therein is incorporated herein by reference.